A variety of techniques are used to stack packaged integrated circuits. Some methods require special packages, while other techniques stack conventional packages.
The predominant package configuration employed during the past two decades has encapsulated an integrated circuit in a plastic surround typically having a rectangular configuration. The enveloped integrated circuit is connected to the application environment through leads emergent from the edge periphery of the plastic encapsulation. Such “leaded packages” have been the constituent elements most commonly employed by techniques for stacking packaged integrated circuits.
Leaded packages play an important role in electronics, but efforts to miniaturize electronic components and assemblies have driven development of technologies that preserve circuit board surface area. Because leaded packages have leads emergent from peripheral sides of the package, leaded packages occupy more than a minimal amount of circuit board surface area. Consequently, alternatives to leaded packages known as chip scale packages or “CSPs” have recently gained market share.
A commonly used style of CSP provides connection to a packaged integrated circuit through a set of contacts (often embodied as “bumps” or “balls”) arrayed across a major surface of the package. Instead of leads emergent from a peripheral side of the package, contacts are placed on a major surface and typically emerge from the bottom surface of the package.
The absence of “leads” on package sides renders most of the conventional stacking techniques devised for leaded packages inapplicable for CSP stacking. Frequently, CSP stacking provides one or more flex circuits interconnecting the contacts of respective CSPs. Also, CSP stacking may more often dispose one CSP bonded to another CSP of the stack. Conventional stacking techniques devised for leaded packages also often are inadequate for stacking integrated circuits packaged in different forms, such as a stack comprising both CSPs and leaded packages.
A variety of previous techniques for stacking CSPs and mixed integrated circuit packages may present complex assembly problems. Therefore, a technique and system is needed for stacking CSPs that provides a thermally-efficient, reliable structure allowing efficient production at reasonable cost with readily understood and managed materials and methods.